Quality Control Pathways for Nucleus-Encoded Eukaryotic tRNA Biosynthesis and Subcellular Trafficking

Abstract

tRNAs perform an essential role in translating the genetic code. They are long-lived RNAs that are generated via numerous posttranscriptional steps. Eukaryotic cells have evolved numerous layers of quality control mechanisms to ensure that the tRNAs are appropriately structured, processed, and modified. We describe the known tRNA quality control processes that check tRNAs and correct or destroy aberrant tRNAs. These mechanisms employ two types of exonucleases, CCA end addition, tRNA nuclear aminoacylation, and tRNA subcellular traffic. We arrange these processes in order of the steps that occur from generation of precursor tRNAs by RNA polymerase (Pol) III transcription to end maturation and modification in the nucleus to splicing and additional modifications in the cytoplasm. Finally, we discuss the tRNA retrograde pathway, which allows tRNA reimport into the nucleus for degradation or repair.

INTRODUCTION

The tRNAs, in contrast to mRNAs, are long-lived molecules with half-lives reported to be from hours to days (1–4). tRNAs serve the essential function of delivering amino acids specified by mRNA codons to the protein synthesis machinery. tRNAs also have numerous alternative functions such as serving as primers for retroviral replication, signaling the general amino acid control pathway, and binding cytochrome c to regulate apoptosis (4, 5). Thus, it is exceedingly important that tRNAs are appropriately structured, processed, and modified.

The intent of this minireview is to provide a description of the quality control pathways that ensure that eukaryotic tRNAs, transcribed in the nucleus, are appropriately structured, processed, and modified and are delivered to the cytoplasm, where translation occurs. Most of the tRNA quality control pathways have been discovered using the budding yeast (Saccharomyces cerevisiae) model system, and so this review focuses on yeast; however, where data are available, we provide information for other organisms. Some of the quality control pathways have been known for quite some time, but others have been discovered only recently. Surprisingly, each pathway monitors only a subset of tRNA biosynthesis or processing or trafficking events; thus, the pathways have overlapping as well distinct roles in tRNA quality control and they work together to ensure that tRNAs are appropriately structured, processed, and modified. Some of the processes we describe have also been reviewed recently by others (6). This minireview does not intend to describe the aspects of quality control that monitor tRNA function, such as those monitoring correct tRNA aminoacylation and tRNA occupancy on ribosomes; recent reviews by others have addressed such quality control mechanisms (7–9).

In all eukaryotes, scores of gene products are required to produce the pool of mature functional cytoplasmic tRNAs. For example, >100 gene products are required for tRNA processing, modification, and subcellular dynamics in budding yeast (4); surprisingly, even in this well-studied model system, additional gene products involved in tRNA biosynthesis continue to be discovered (see, e.g., reference 10).

Eukaryotic precursor tRNAs (pre-tRNAs) undergo numerous posttranscriptional processing events as they move from their site of transcription in the nucleus to their site of function in translation in the cytoplasm. Initial tRNA transcripts contain 5′ leader and 3′ trailer sequences that are removed, respectively, by the 5′ endonuclease ribonucleoprotein complex, RNase P, and by the 3′ endonuclease, RNase Z (11), and the 3′-to-5′ exonuclease, Rex1, with participation of the RNA binding protein, La (12). Following removal of the 5′ leaders and 3′ trailers, the ubiquitous CCA nucleotides are added to 3′ termini (13). Moreover, posttranscriptional steps also add numerous modifications to the tRNA primary sequences. Various tRNA modifications are added to tRNAs in the nucleus, whereas others are added in the cytoplasm. There are 25 different nucleoside modifications reported for yeast tRNAs, and the average yeast tRNA contains ∼11 to ∼13 modified nucleosides (14). While some particular modified nucleosides serve important roles in accurate decoding, maintenance of reading frames, tRNA identity, responses to environmental stresses, and/or appropriate tRNA tertiary structures (14–18), the functions of other modifications remain unknown (14).

Some tRNA genes contain introns that must be removed from pre-tRNAs posttranscriptionally. The percentages of tRNAs that are encoded by intron-containing genes range from 0% in bacteria to 5% in insects and vertebrates to 22% in budding yeast and to up to 100% in some archaeal organisms (19). Vertebrate tRNA splicing takes place in the nucleus, but yeast pre-tRNA splicing occurs in the cytoplasm, on the surface of mitochondria (20, 21). Nevertheless, the catalytic process for intron removal is conserved between yeast and vertebrates, as tRNA intron removal is catalyzed by a heterotetrameric protein complex, SEN, in both organisms (22, 23). Indeed, removal of introns from pre-tRNAs via the activity of protein-mediated endonucleases appears to be conserved throughout evolution (24). In contrast to the conserved mechanism for removal of introns, the steps to ligate the resulting 5′ and 3′ exons differ among organisms. In vertebrates and some archaeal organisms, a category of protein ligase directly joins the 3′ phosphate of the 5′ exon to the 5′ hydroxyl of the 3′ exon (25, 26). In contrast, ligation in yeast and plants proceeds via a complicated protein-catalyzed reaction that involves addition of a 5′ phosphate to the 3′ half and joining to the 3′ hydroxyl, 2′ phosphate terminus of the 5′ exon, thereby generating a spliced tRNA with two phosphates at the splice junction (27, 28). The excess 2′ phosphate at the splice junction is then removed by the 2′ phosphotransferase (29, 30).

Finally, mature tRNAs have dynamic movement between the nucleus and the cytoplasm; cytoplasmic tRNAs travel in a retrograde direction to the nucleus via tRNA retrograde nuclear import and then once again access the cytoplasm via tRNA reexport (31–34). In sum, successful execution of tRNA biosynthesis requires coordination of scores of diverse enzymes that are located in separate subcellular compartments.

Fidelity in tRNA biosynthesis and subcellular trafficking theoretically could be achieved if each of the enzymes involved in tRNA posttranscriptional processing and modification were highly accurate and if cells were to possess multiple tRNA nuclear exporters, each dedicated to one of the multiple different tRNA sequences (42 in yeast). However, cells have not evolved such accurate mechanisms for tRNA production. Rather, the biosynthesis and cell biology machineries are error prone and the errors are corrected or eliminated by a surprisingly large number of quality control mechanisms, some of which compete for the same substrates. We provide a description of each of the known six mechanisms that check tRNA structure and correct or destroy aberrant tRNAs. The mechanisms are arranged in order so as to follow the tRNAs from the initial transcript, through the various processing steps in the nucleus, to steps that occur in the cytoplasm, and, finally, to the tRNA retrograde pathway by which cytoplasmic tRNAs imported into the nucleus and then again reexported to the cytoplasm (Fig. 1 and Table 1).

FIG 1.

Overlapping layers of nucleus-encoded tRNA control pathways. (A) Quality control pathways for tRNAs encoded by intron-lacking genes. (B) tRNA quality control pathways encoded by intron-containing genes. tRNAs are shown as the cloverleaf structure with color-coded circles. Purple circles depict transcribed leader and trailer sequences at the 5′ and 3′ ends, respectively. Blue circles depict the mature exons. Green circles depict the posttranscriptionally added CCA nucleotides. A black circle indicates a modification known to occur on initial pre-tRNA transcripts. Orange, brown, and open circles indicate various posttranscriptional modifications. Yellow circles in panel B depict intron sequences. The canonical tRNA biosynthesis pathway and subcellular traffic patterns are indicated with black arrows. Red dotted arrows indicate aberrant pre-tRNAs that are processed too slowly at 3′ ends and are targeted to nuclear exosome. The green dotted arrow in panel A indicates that aberrant pre-tRNA^Met missing m¹A₅₈ is targeted to a nuclear exosome. Blue dotted arrows indicate premature nuclear export that occurs before 5′ and 3′ processing. Aberrant transcripts are returned to the nucleus via constitutive retrograde nuclear import and are either processed or degraded. Magenta and purple dotted arrows indicate premature nuclear export before complete modification; hypomodified tRNAs are indicated by magenta or purple circles and arrows. Hypomodified tRNAs may return to the nucleus via retrograde import or may be destroyed by the cytoplasmic RTD exonuclease, Xrn1. Imported hypomodified tRNAs are repaired or degraded by the Rat1 nuclear RTD exonuclease. Hypomodified tRNAs and/or tRNAs with improper structure may be marked by Cca1 to generate CCACCA 3′ termini and then degraded by RTD nucleases.

TABLE 1.

tRNA quality control pathways

Quality control pathway	Gene product(s)	Substrate(s)	Localization
3′–5′ degradation	Nuclear exosome	Aberrant pre-tRNA	Nucleus
CCA or CCACCA addition	tRNA nucleotidyl transferase	Unspliced, spliced/mature tRNA, aberrant tRNA	Nucleoplasm, cytoplasm, mitochondria
Nuclear aminoacylation	Aminoacyl-tRNA synthetases	Mature tRNA with 3′ CCA	Nucleus
tRNA nuclear export and reexport	Los1/exportin-t	Unspliced, spliced/mature tRNA	Primarily nucleus
	Msn5/exportin-5	Spliced, aminoacylated tRNA	Primarily nucleus
5′–3′ degradation-RTD	Rat1	tRNA with aberrant modification and/or structure	Nucleus
	Xrn1	tRNA with aberrant modification and/or structure	Cytoplasm
tRNA retrograde nuclear import	Mtr10? Others?	Cytoplasmic tRNA, including normal and aberrant tRNA	Primarily cytoplasm

Nuclear exosome.

The nuclear exosome is a 3′ to 5′ exonuclease. It degrades precursor tRNAs with aberrant structure or pre-tRNAs with 3′ termini that are processed too slowly. For example, yeast ${\text{pre-tRNA}}_{\text{i}}^{\text{Met}}$ is aberrantly structured and unstable if it is missing m¹A₅₈ due to mutations of TRM6 or TRM61 that encode the dimeric tRNA m¹A₅₈ methyltransferase (1, 2). Defects in TRM6 or TRM61 cause growth defects that can be suppressed by mutations of RRP44, which encodes a component of the nuclear exosome, and TRF4, which encodes a nonconventional poly(A) polymerase (35, 36). Genetic and biochemical studies of the suppressors led to the current understanding that aberrant pre-tRNAs that are either hypomodified or are processed too slowly at 3′ ends first acquire a 3′ oligo(A) sequence via action of Trf4 and are then targeted to the nuclear exosome, thereby destroying the aberrant pre-tRNAs prior to nuclear export (37–39) (Fig. 1 and Table 1).

CCA addition by nucleotidyl transferase.

All tRNAs possess the CCA nucleotide sequence at 3′ termini. This CCA sequence is essential for tRNA aminoacylation. In eukaryotes, the CCA nucleotides are generally not encoded by tRNA genes; rather, they are added posttranscriptionally by tRNA nucleotidyl transferase. The budding yeast CCA1 gene encodes multiple isoforms of tRNA nucleotidyl transferase that are differentially located in the nucleoplasm, cytoplasm, and mitochondrial matrix (40). The nucleoplasmic isoform generates mature tRNA 3′ termini that are required for efficient nuclear tRNA aminoacylation and tRNA nuclear export, whereas the cytoplasmic isoform functions in repair of damaged tRNA 3′ termini (13, 41). The mitochondrial pool serves to add the CCA sequence to the tRNAs encoded by the mitochondrial genome (40). Studies in Escherichia coli demonstrated that the CCA sequence protects tRNA 3′ termini from promiscuous attack by tRNA 3′ processing enzymes (42). However, recent data for vertebrate cells have shown that the 3′ CCA sequence is removed by endonuclease angiogenin upon oxidative stress, thereby rapidly repressing translation. Upon removal of the environmental stress, tRNA nucleotidyl transferase restores CCA termini, allowing aminoacylation and protein synthesis to ensue (43). tRNA nucleotidyl transferase can also mark particular tRNAs that are hypomodified and/or possess aberrant tertiary conformations via extended polymerization to generate CCACCA 3′ termini, and these CCACCA-containing tRNAs are subsequently degraded (44, 45) (see below and Fig. 1 and Table 1). Thus, tRNA nucleotidyl transferase functions in tRNA aminoacylation, tRNA repair, protection of 3′ termini, stress response, export of tRNAs from the nucleus to the cytoplasm, and destruction of aberrantly structured tRNAs.

tRNA nuclear aminoacylation.

Protein synthesis in the cytoplasm requires continuous cytoplasmic tRNA aminoacylation as amino acids are transferred from charged tRNAs to growing polypeptide chains on ribosomes. So, the discovery that tRNA aminoacylation also occurs in the nucleus of Xenopus oocytes was unanticipated (46). The existence of aminoacyl-tRNA synthetase nucleoplasmic pools is conserved from yeast to mammals (4). Studies of both yeast and Xenopus oocytes demonstrated that defects in tRNA charging in the nucleus, resulting from conditional mutations or inhibition of aminoacyl-tRNA synthetases, reduce the efficiency of tRNA nuclear export (46–48). As the aminoacyl-tRNA synthetases check appropriate tRNA 3′ ends and internal sequences, tRNA aminoacylation in the nucleus provides an additional level of tRNA quality control mechanism. Nuclear pools of aminoacyl-tRNA synthetases and their spliced variants also can serve additional functions distinct from tRNA aminoacylation (49, 50).

β-Importin family members—exportin-t/Los1 and Msn5.

The β-importin family member exportin-t (vertebrates)/Xpot (fission yeast)/Los1 (budding yeast)/PAUSED (plants) exports tRNA from the nucleus to the cytoplasm (47, 51–53). This tRNA nuclear exporter also serves an important role in tRNA quality control as it monitors tertiary tRNA structure as well as appropriately processed tRNA 5′ and 3′ termini. In vitro binding studies as well as studies to assess tRNA nuclear export employing Xenopus oocytes documented that exportin-t preferentially binds tRNAs with the appropriate tertiary structure, although it has no preference between intron-containing and spliced tRNAs. Importantly, tRNAs with mature 5′ termini and mature 3′ CCA-containing ends preferentially bind exportin-t (54, 55). Since, in vertebrates, pre-tRNA splicing precedes end processing (46), preferential interaction of exportin-t with appropriately structured tRNAs with mature termini serves as a key quality control mechanism to deliver spliced, end-matured, and appropriately structured tRNAs to the cytoplasm. Similarly, in vivo studies in yeast demonstrated that mutant tRNAs are inefficiently exported to the cytoplasm (56). Finally, structural studies at 3.2-Å resolution for the fission yeast exportin-t, Xpot, in complex with RanGTP and tRNA elegantly demonstrated that Xpot binds the tRNA backbone and mature CCA termini (57). The high-resolution Xpot structure is compatible with its binding of aminoacylated tRNAs (57) (Fig. 1A). Indeed, recent in vivo analyses of Los1-tRNA-RanGTP nuclear export complexes demonstrated that Los1 can form export complexes with both charged tRNA and uncharged tRNA (58).

In vertebrate cells, exportin-t plays the major role in tRNA nuclear export (54, 55). However, another β-importin family member, exportin-5, has been implicated in tRNA nuclear export (4, 59, 76, 77). The major role for exportin-5 in vertebrates and plants is to export pre-microRNA from the nucleus to the cytoplasm (60, 61). However, exportin-5 has an alternative minor role in delivery of tRNAs to the cytoplasm in vertebrate cells (59, 76, 77). The yeast exportin-5 homologue is Msn5. A well-established role for Msn5 is to export particular phosphorylated proteins from the nucleus to the cytoplasm (62). Msn5 also has a second function in tRNA nuclear export as msn5Δ cells accumulate nuclear pools of tRNA and msn5Δ los1Δ double mutants exhibit larger nuclear pools than either single mutant (33, 63). Los1 and Msn5 have only partially overlapping functions. Los1 exports unspliced or spliced tRNAs, regardless of the aminoacylation status. In contrast, Msn5 preferentially exports mature aminoacylated tRNAs in the tRNA reexport step (see below). To specifically and efficiently export aminoacylated tRNA to the cytoplasm, Msn5 forms a quaternary complex with nuclear pools of translation elongation factor 1α (eEF1A), RanGTP, and aminoacylated tRNA (58). Since such aminoacylated tRNAs have first been proofread by the nuclear pools of aminoacyl-tRNA synthetases, Msn5 also contributes to tRNA quality control. Thus, both tRNA nuclear exportins function in tRNA quality control prior to tRNA nuclear export, but they employ somewhat different mechanisms to recognize appropriate tRNAs.

Nuclear and cytoplasmic 5′-to-3′ decay by the RTD mechanism.

Yeast tRNAs missing combinations of particular modifications are unstable at elevated temperatures, resulting in temperature-sensitive growth. The temperature-sensitive growth and tRNA instabilities are partially suppressed by mutations of either the nucleus-located 5′-to-3′ exonuclease, Rat1, or its cytoplasmic counterpart, Xrn1, or by mutation of MET22, which causes accumulation of a byproduct which inhibits both Rat1 and Xrn1 (64, 65). Thus, Rat1, Xrn1, and Met22 define components of the rapid tRNA decay (RTD) quality control pathway that destroys tRNAs missing particular sets of modifications. Further studies to investigate the specificity of this RTD pathway demonstrated that the pertinent tRNA modifications contribute to tRNA stability of the acceptor and T stems of the relevant substrates (66). Moreover, a recent unbiased genome-wide, high-throughput analysis assessing the consequences of mutations of tRNA^Tyr with respect to its role in translation unexpectedly showed that mutations throughout the tRNA body cause instabilities that are rescued in met22Δ strains, documenting that the RTD nucleases detect conformational changes of tRNAs that are distant from the site of catalysis at 5′ termini (67). Although the existence of an RTD-like mechanism in metazoans has not been thoroughly explored, there is at least one report demonstrating 5′-to-3′ tRNA degradation mediated by the nucleoplasmic Xrn2 (yeast Rat1) upon heat stress of mammalian cells (68). In sum, the data document that the RTD 5′-to-3′ nucleases serve as a quality control mechanism that destroys hypomodified tRNAs and aberrantly structured tRNAs (66, 67).

tRNA retrograde pathway.

The essential function for tRNA is to deliver amino acids to growing polypeptides during translation in the cytoplasm. Therefore, tRNA subcellular trafficking was considered to be a one-way movement from the nuclear site of biosynthesis to the cytoplasmic site of protein synthesis. However, it is now known that tRNA subcellular traffic is bidirectional; tRNAs in the cytoplasm travel in a retrograde direction to the nucleus via tRNA retrograde nuclear import and then once again access the cytoplasm via tRNA reexport (31, 33, 34). This tRNA retrograde pathway is conserved from budding yeast to vertebrate cells (4). A key issue is that of why tRNAs move from their cytoplasmic site of function back to the nucleus. To date, four novel roles for the tRNA retrograde process have been discovered: modification of tRNA^Trp G₃₇ to yW₃₇ in budding yeast (69) and efficient translation of particular yeast mRNAs involved in methionine and arginine biosynthesis (70), as a mechanism to deliver retrotranscribed HIV genomes to the nucleus (71) and, relevant to this article, as a tRNA quality control mechanism (72) (Fig. 1).

Although, as described above, the exportin-t/Xpot/Los1 tRNA exportin monitors overall tRNA structure and mature termini, recent evidence shows that it sometimes erroneously delivers tRNAs that have not been completely processed to the cytoplasm. Since, in budding yeast, tRNA splicing occurs in the cytoplasm, splicing serves as a proxy for tRNA nuclear export. Interestingly, there are low levels of spliced tRNA that possess unprocessed 5′ and 3′ termini even in wild-type cells (72). These data demonstrate that tRNA transcripts that have not been subjected to end processing can reach the cytoplasm to be spliced. Moreover, the quantity of such aberrant end-containing spliced pre-tRNAs in the cytoplasm increases upon overexpression of Los1 (72). Similarly, some of the tRNA modification enzymes, such as Trm1, which encodes the m²₂G methyltransferase, reside in the nucleus and, therefore, m²₂G modification should be complete before tRNAs exit to the cytoplasm. And yet, low levels of spliced hypomodified m²₂G can be detected in wild-type cells and the levels of such hypomodified tRNAs increase upon Los1 overexpression (72). Thus, the expression level of Los1 in cells must be finely tuned so that the rate of tRNA nuclear export does not compete with completion of the numerous tRNA posttranscriptional processing steps that occur in the nucleus. Even so, the data document that nuclear export by Los1 is not error free.

Errors in tRNA nuclear export that prematurely deliver immature tRNAs to the cytoplasm can be corrected by the constitutive tRNA retrograde import process. The Mtr10 β-importin and the Dhh1 and Pat1 P-body machinery components have been implicated in tRNA nuclear import, as cells with mutations in these genes are unable to accumulate cytoplasmic tRNAs in the nucleus; however, there is no evidence that these proteins directly move tRNA from the cytoplasm to the nucleus (31, 58, 63, 73). Yeast cells harboring mtr10Δ or dhh1Δ pat1Δ mutations possess elevated levels of both end-extended and m²₂G hypomodified spliced tRNAs, supporting the idea of a role for tRNA retrograde nuclear import in tRNA quality control for end maturation and m²₂G modification (72). Whether the tRNA retrograde process serves as a quality control pathway that monitors other tRNA modifications added prior to nuclear export remains to be determined. Aberrant tRNAs constitutively imported into the nucleus may be eliminated either by the action of the nuclear Rat1 RTD exonuclease or perhaps instead by providing “a second opportunity” for iterative appropriate tRNA processing and modification before tRNA is reexported to the cytoplasm. In yeast, both Los1 (exportin-t) and Msn5 (exportin-5) function in tRNA reexport, but Msn5 is apparently dedicated to this process (at least for the category of tRNAs encoded by intron-containing genes) because it preferentially exports spliced, aminoacylated tRNAs from the nucleus to the cytoplasm (58).

Cooperation and competition between tRNA retrograde import and cytoplasmic RTD.

The relationship between the tRNA retrograde import and the cytoplasmic RTD pathways in tRNA quality control has been explored (72). Cells defective in the tRNA retrograde nuclear import, i.e., mtr10Δ or dhh1Δ pat1Δ cells, accumulate both end-extended and hypomodified tRNA. In contrast, xrn1Δ cells missing the cytoplasmic RTD 5′-to-3′ exonuclease accumulate hypomodified tRNA but not end-extended spliced tRNAs (72). Therefore, tRNA nuclear import prevents cytoplasmic accumulation of both aberrantly end-processed and hypomodified tRNAs, whereas the cytoplasmic RTD process monitors tRNA modification and structure but not termini. Thus, tRNA nuclear import and cytoplasmic RTD have partially overlapping roles in tRNA quality control and they compete for some of the same substrates.

Cells defective for either tRNA nuclear import or cytoplasmic RTD (mtr10Δ or xrn1Δ cells) are viable, but cells simultaneously missing both of these quality control pathways (mtr10Δ xrn1Δ cells) are not viable (72). The data support the notion that cells must possess at least one of these pathways for viability; however, as both Mtr10 and Xrn1 serve multiple functions in yeast, it is possible that the lethality of the double mutants results from a reason other than tRNA quality control; future studies are required to distinguish between the possibilities.

PERSPECTIVES

It is surprising that there are at least six separate quality control mechanisms, each partially contributing to the cytoplasmic pools of appropriately structured, processed, and modified tRNAs. Why these Rube Goldberg machine-like mechanisms for tRNA quality control evolved is not known. However, similar multiple layers of quality control mechanisms also have evolved for other steps of translation. For example, there are three separate mechanisms (triple sieve) that function in accurate aminoacylation of bacterial tRNA^Pro (74). Many questions remain. For instance, it is unknown whether aberrant tRNAs imported to the nucleus via the retrograde process are repaired by nuclear tRNA posttranscriptional enzymes and/or whether they are degraded via the exosome or the Rat1 nuclear RTD enzyme. Moreover, recent studies showed that the extent of particular tRNA modifications is altered by environmental stresses (15–17, 75), and it will therefore be interesting to learn how the resulting hypomodified tRNAs generated under such conditions escape the RTD quality control pathway. Finally, it is quite possible that an additional quality control mechanism(s) for tRNA production, subcellular distribution, and tRNA roles in protein synthesis has yet to be discovered.

Biographies

Anita K. Hopper received a Ph.D. degree from the University of Illinois and conducted postdoctoral studies at the University of Washington. She held faculty positions at the University of Massachusetts Medical Center, the Pennsylvania State University College of Medicine, and Ohio State University, where she is a professor of molecular genetics. She is a Fellow of the American Academy of Microbiology and American Association for the Advancement of Science (AAAS) and received a lifetime achievement award from the RNA Society. She is a past secretary of the Genetics Society of America and a past president of the RNA Society. She was on the Molecular and Cellular Biology (MCB) Editorial Board from 1986 to 1989 and served as an associate editor from 1989 to 2000. Her laboratory has been studying tRNA biology since the 1970s. The studies have uncovered numerous gene products involved in tRNA nuclear-cytoplasmic dynamics, tRNA processing, and tRNA intron turnover. A key discovery was the paradigm-shifting tRNA retrograde pathway by which cytoplasmic tRNAs accumulate in the nucleus. This process has previously unappreciated roles in tRNA quality control.

Hsiao-Yun Huang received an M.S. degree from National Central University Taiwan and a Ph.D. degree from The Ohio State University. She is currently conducting postdoctoral studies at Howard Hughes Medical Institute, Indiana University. She is a member of the Genetics Society of America and the RNA Society. She has studied translation initiation and tRNA biology since 2003. Her Ph.D. studies with Dr. Hopper uncovered connections between β-importin family members, tRNA subcellular trafficking, and nutrient status. She developed a novel in vivo biochemical method, and, using this methodology, she uncovered distinct roles for β-importin family members in the paradigm-shifting tRNA retrograde pathway by which cytoplasmic tRNAs are imported to the nucleus and are then reexported to the cytoplasm. Her work has contributed to understanding regarding how tRNA nuclear reexport contributes to tRNA quality control.